Currently, size of a flyback switching power source is mainly determined by size of the transformer and the input filtering capacitor in the power source, and a typical small switching power source adopts a relatively high switching frequency and a relatively low primary peak current, in which the relatively high switching frequency can ensure that the transformer has a relatively high power density, and the relatively low primary peak current can ensure that core saturation does not occur when a transformer having a small core area is used.
Currently, most flyback switching power sources work in a DCM (Discontinuous Inductor Current) mode, i.e., next charging is started only when a primary inductor is charged fully, as shown in FIG. 1, in which IP is the primary peak current and T is an operational period of the system. In order to ensure that the switching power source is working in the DCM mode and has an appropriate magnetic flux density, the inductance should be relatively small in high frequency applications. Thus, such a small switching power source generally has following defects.
1. The inductance is small, and a relatively large air gap exists between magnetic cores (in a condition of small core area), thus causing a large iron loss and being unfavorable for transformer manufacturing.
2. When a high alternating current is inputted, the high switching frequency causes a substantial switching loss, which greatly influences the efficiency of the switching power source.